1. Field of the Invention
This invention relates to a road surface condition sensor for judging if there is an appreciable unevenness on a road surface to provide desirable performance of brakes for a vehicle.
2. Description of the Related Art
An antilocking system for the brakes for a vehicle, such as a motorcar or a motorcycle, is known when a vehicle is traveling on a road, a braking force working between a braked wheel and the road surface generally increases as a slippage ratio of the wheel increases. Here, the slippage ratio of the wheel is defined as the ratio of the difference between the running speed of the vehicle and the peripheral speed of the wheel vs. the running speed of the vehicle. However, when the slippage ratio attains and exceeds a critical value, the braking force decreases from its maximum. The antilocking control of the brakes makes use of the above-mentioned relationship between the slippage ratio and the braking force to provide an optimum braking performance for the vehicle.
In the relationship between the slippage ratio and the braking force, the critical value of the slippage ratio (a "target slippage ratio") which gives the maximum of the braking force varies depending on the coefficient of friction of the road surface against the wheel. It is therefore important for the antilocking control of brakes to accurately estimate the coefficient of friction of the road surface, while the vehicle is running, to determine a proper target slippage ratio.
No matter what means to find out the coefficient of friction of a road surface is employed, it is necessary to obtain accurate values of the rotation of wheels, the speed and/or acceleration of the vehicle, etc., so as to accurately estimate the coefficient of friction.
However, as every part of a vehicle during operation is always vibrated by unevenness of the road surface, noises due to the vibration tend to mingle with the output of various sensors for detecting quantities necessary to obtain the coefficient of friction, causing efforts to accurately determine the coefficient of friction and to set a proper target slippage ratio to fail.